1. Field of the Invention
The present invention relates to a method of isolating semiconductor devices enabling to prevent an active area from being reduced due to the increase of an isolation area by means of forming trenches.
2. Discussion of the Related Art
As the integration of semiconductor device has been developed, so the technology for reducing isolation areas occupying great portions in semiconductor device has been pursued for actively.
In general, semiconductor devices have been isolated from one another by LOCOS that is Local Oxidation of Silicon. LOCOS is the process as following. A pad oxide film is formed between a nitride film and a semiconductor substrate in order to lessen the stress caused by the different characteristic of heat expansion of the substrate and the nitride film which is formed on the active area and used as a hard mask. Then, a field area on which no nitride film is formed is oxidized to form a field oxide film used as an isolation area. The field oxide film grows under the edge of a nitride film pattern horizontally as well as vertically since oxidant diffuses along with the pad oxide film.
Such a phenomenon is so-called a Bird's Beak in that the field oxide film encroaches on the active area. The length of the bird's beak is half a thickness of the field oxide film. Hence, the length of bird's beak is to be minimized in order not to decrease the active area.
Although a method of decreasing the thickness of the field oxide film to reduce the length of bird's beak has been introduced, a speed of delivering a signal is lowered due to the increase of parasitic capacitance between a conductive line and semiconductor substrate over 16M DRAM. Moreover, the characteristic of isolation between devices deteriorates due to the reduced threshold voltages Vt of parasitic transistors formed in the isolation area between devices by a conductive line used as a gate of each device.
Accordingly, methods of improving the isolation characteristics have also been developed to reduce the length of bird's beak such as PBLOCOS(Poly Si Buffeted LOCOS) inserting a polysilicon layer between a semiconductor substrate and a nitride film and reducing the thickness of a pad oxide film for buffering stress, SILO(Sealed Interface LOCOS) protecting a sidewall spacer of a pad oxide film and Recessed Oxide LOCOS forming a field oxide film in a semiconductor substrate.
Unfortunately, such methods as mentioned above are no longer appropriate for a method of isolating devices of next generation having integrity over 256M DRAM due to a precise design rule and the planarity of a surface of the isolation area.
Accordingly, a method of box typed trench isolation having buried oxide has been developed to overcome the problems having been caused by the conventional arts. Trench Isolation has a structure of a buried oxide film formed by CVD(Chemical Vapor Deposition) after a trench has been formed in a semiconductor substrate. Thus, the loss of an active layer is prevented from bird's beak. And an even surface is attained by means of filling the trench with oxide and etching back the oxide.
FIG. 1A to FIG. 1D show cross-sectional views of isolating semiconductor devices according to a conventional art.
Referring to FIG. 1A, a first oxide film 13 is formed on a semiconductor substrate 11 by heat oxidation. Then, a nitride film 15 is formed on the first oxide film 13 by CVD. An isolation and active area are defined by means of removing certain parts of the first oxide film 13 and the nitride film 15 selectively by photolithography to expose a surface of the semiconductor substrate 11. A first and second trench 17 and 18 having different sizes are formed by dry etching the exposed isolation area of the semiconductor substrate 11 to have both a certain inclination angle and a certain depth in use of the remaining nitride film 15 as a hard mask.
Referring to FIG. 1B, a second oxide film 19 is formed by thermally oxidizing the inner surfaces of the first and second trenches 17 and 18, wherein the second oxide film 19 compensates for the damages caused by the formation of the trenches such as lattice defects formed inner surfaces of the trenches 17 and 18 and is not formed on the surface of the nitride film 15.
A filling oxide 21 is formed on the nitride film 15 by CVD to fill up the trenches 17 and 18 with silicon oxide, wherein the filling oxide is well deposited in the first trench 17 having a low aspect ratio due to its wide size. On the contrary, a void 22 is formed inside the filling oxide 21 in the second trench 18 having a high aspect ratio due to its small size, wherein the void 22 is caused by the deposition characteristics of the filling oxide 21 and lies on the level of the surface of the silicon substrate 11.
Referring to FIG. 1C, a field oxide film 23 is formed by means of removing a portion of the filling oxide 21 having been formed only on the nitride film 15 to remain inside the trenches 17 and 18 by CMP(Chemical Mechanical Polishing), whereby the void 22 in the filling oxide 21 is disclosed.
Referring to FIG. 1D, the nitride film 15 and the first oxide film 13 are removed in turn by wet etch, wherein the surface of the first oxide film 13 remaining in the trenches 17 and 18 is also removed to be planar with the surface of the semiconductor substrate 11.
However in the above-mentioned conventional method of isolating semiconductor devices, the middle part of the field oxide film filling up the first trench having a high aspect ratio caves in due to the void, and then its size increases on removing the first oxide film, etc.
On forming a gate, polysilicon remains on the middle part of the field oxide to short. Parasitic capacitance between the semiconductor substrate and the conductive line such as a wordline and a bitline and the like also increases due to the decrease of the thickness of the field oxide film by the middle part.
Moreover, the characteristics of isolation is lowered and the Latch-Up appears when the void is prevented by means of forming a shallow trench or a trench having a small angle of inclination in order to fill up the trench with ease.